This invention pertains generally to continuous wave (CW) radar systems and particularly to a signal processing technique for processing a sine wave frequency modulated (FM) CW waveform to obtain unambiguous target range and range rate in a single radar dwell on a target.
As is known in the art, CW radars have the advantage of being able to detect targets at any range with little, if any, chance of velocity ambiguity. This capability has led to the use of CW radars in anti-aircraft missile systems for detecting low altitude targets where return signals from such targets compete with large stationary clutter returns. In such systems which are presently used, target illumination and target tracking are performed, after a particular target has been detected by a first CW radar (or CW acquisition radar), by a second CW radar (or tracking/illuminating radar). The latter types of CW radars have exceedingly low noise sidebands which permit target tracking even under severe clutter conditions.
Obviously, if the tracking/illuminating radar could be modified to derive target range and range rate in a single dwell, such a radar could perform the initial target acquisition function and thereby eliminate the need for a separate CW acquisition radar. However, to obtain range and range rate information with a CW radar necessarily implies that the CW waveform must be modulated in some manner; and a modulated waveform in turn implies that range and Doppler ambiguities may occur. Furthermore, some consideration must be given to the effects of stationary clutter. That is to say, because the 3 dB width of the ambiguity function along the range axis is a function of the transmitted bandwidth, returns from stationary clutter will have a spectral width equal to the transmitted bandwidth centered at zero Doppler frequency. Thus, returns from stationary clutter can seriously degrade the ability of a system employing a modulated waveform to detect slowly moving targets.
One known method of encoding a CW waveform is through the use of a pseudo-random coded (PRC) bi-phase modulation described in U.S. Pat. No. 4,042,925 to Albanese et al. In general, however, the use of broadband waveforms such as those contemplated in the cited reference requires the use of a receiver with a wide dynamic range to process the radar return signals because clutter filtering cannot be accomplished prior to correlation processing. Furthermore, to avoid the range ambiguity problem associated with PRC binary coded waveforms, a rather complex signal processor is required.